1. Field of the Invention
The present invention relates to a conductive paste for forming terminal electrodes of a monolithic ceramic electronic component such as a monolithic capacitor or a monolithic positive temperature coefficient (PTC) thermistor. It also relates to a monolithic ceramic electronic component using the conductive paste and to a method for making the monolithic ceramic electronic component.
2. Description of the Related Art
Conventionally, internal electrodes of a monolithic ceramic capacitor, i.e., an example of a monolithic ceramic electronic component, have been made of silver, silver-palladium, or the like. However, since these materials are expensive, nickel, which is a relatively inexpensive base metal, has replaced these materials.
On the other hand, terminal electrodes of a monolithic ceramic capacitor are composed of silver, which exhibits superior conductivity and can be baked at low temperatures. The silver terminal electrodes are coated with a nickel layer and then a tin layer or a solder layer to improve the solderability and to obtain a monolithic ceramic capacitor.
The internal electrodes of a monolithic PTC thermistor, which is another example of a monolithic ceramic electronic component, are made of nickel which makes an ohmic contact to a ceramic, i.e., an n-type impurity semiconductor. A PTC thermistor having a desired positive resistance temperature characteristic is made by simultaneously baking stacked ceramic green sheets and internal electrodes in a reducing atmosphere and subsequently forming terminal electrodes by baking in air while reoxidizing the ceramic.
However, nickel and silver do not form a solid solution. Thus, a monolithic ceramic capacitor having nickel internal electrodes and silver terminal electrodes suffers from a problem in that the bonding of the internal electrodes to the terminal electrodes is difficult, thereby often failing to obtain a desired capacitance.
In view of the above problem, copper which forms a complete solid solution with nickel has drawn much attention as a material of the terminal electrodes. However, since copper is easily oxidized, baking of the copper-containing conductive paste to form terminal electrodes must be performed in a reducing atmosphere. Such a process increases the manufacturing cost. Moreover, since the oxygen concentration in the reducing atmosphere is low, the decomposition rate of the vehicle contained in the conductive paste is also low. As a result, the characteristics of the product may be affected by the residual carbon.
As with the monolithic ceramic capacitor, a monolithic PTC thermistor having terminal electrodes composed of silver, which can be baked in air, suffers from a problem in that silver does not form a solid solution with nickel in the internal electrodes, thereby inhibiting bonding of the internal electrode to the terminal electrode. Other examples of the terminal electrode paste that can be baked in air include an aluminum paste and a zinc paste. However, electrodes made of aluminum or zinc have poor solderability and suffer from the problem that aluminum or zinc in the electrode elutes into a plating solution when electrolytic plating is performed onto the electrodes.